The present invention relates to a sequential control process for a matrix display using the cholesteric-nematic phase transition effect of a liquid crystal. It is used in the construction of liquid crystal displays, which are more particularly employed in the binary display of complex images or in the display of alphanumeric characters.
More specifically, the invention relates to the control of a matrix display incorporating a display cell constituted by two transparent insulating walls and by a liquid crystal having matrix-distributed areas and inserted in a cross-bar system.
FIG. 1 shows such a matrix display, which comprises a display cell having two generally transparent walls 10 and 12, arranged on either side of an insulating material shim 14, defining a volume 16 which is occupied, when the cell is fitted, by a liquid crystal film. Two systems of electrodes, each constituted by a series of semitransparent, conductive, parallel strips are dposited on walls 10 and 12. The rows of electrodes e.g. having a number p are designated x.sub.i, in which i is an integer which can assume all values between 1 and p, and the columns of electrodes, e.g. having a number q which are designated y.sub.j, in which j is an integer, can assume all values between 1 and q.
Thus, the useful surface of the liquid crystal is broken down into a mosaic of areas corresponding to the overlap areas of two systems of electrodes, each area corresponding to the overlap of two strips x.sub.i and y.sub.j and which therefore can be designated x.sub.i y.sub.j. The rows and columns of electrodes can carry electric signals suitable for exciting the liquid crystal, which has an optical property dependent on said excitation.
In the invention, the sensitization of an area of the liquid crystal takes place by applying to electrodes x.sub.i and y.sub.j electrical voltages, which lead to the appearance of an electric field within the liquid crystal. This electric field makes it possible to act on the cholesteric-nematic phase transition of the liquid crystal. The successive sensitization of the areas, in accordance with the known sequential control principles, makes it possible to make an image or picture appear on the complete cell by defining it point by point.
The operation of such a display will briefly be described. The liquid crystal has two threshold voltages, a low threshold voltage V.sub.B and a high threshold voltage V.sub.H, such that 0&lt;V.sub.B &lt;V.sub.H. The applcation of a potential difference between the rows x.sub.i and the columns y.sub.j, or control voltage, which exceeds the high threshold voltage V.sub.H, makes it possible to obtain the liquid crystal in nematic form and the application of a potential difference between the rows x.sub.i and the columns y.sub.j which is lower than the low threshold voltage V.sub.B makes it possible to obtain the liquid crystal in cholesteric form, no matter what the preceding phase of the liquid crystal. The obtaining of a nematic phase for an area x.sub.i y.sub.j of the liquid crystal corresponds to the display of this area, which becomes white in the presence of a dichroic dye, and the obtaining of a cholesteric phase for said same area corresponds to the undisplayed state of said area, which then appears black due to the dichroism of the dye.
In addition, this type of display cell has a certain memory effect. Thus, after obtaining the displayed state of area x.sub.i y.sub.j, the application of a potential difference between row x.sub.i and column y.sub.j between voltages V.sub.B and V.sub.H is sufficient to maintain the displayed state of said area. In the same way, after obtaining the undisplayed state of area x.sub.i y.sub.j, the application of a potential difference between row x.sub.i and column y.sub.j (between voltages V.sub.B and V.sub.H) is sufficient to maintain the undisplayed state of this area. It should be noted that these maintaining voltages of the displayed or undisplayed states are necessary for maintaining a good contrast between the displayed areas or white points and the undisplayed areas or black points. The absence of these maintaining voltages leads to a significant reduction in this contrast.
FIG. 2a shows the potential difference between row x.sub.i and column y.sub.j, or control voltage V.sub.C, as a function of time, whilst in FIG. 2b, it is possible to see the response curve of the liquid crystal as a function of the value of the potential difference V.sub.C, and response curve corresponding to the light intensity (I) transmitted by area x.sub.i y.sub.j as a function of time. The level portions 20 and 22 of the response curve of the cell correspond to the undisplayed state of area x.sub.i y.sub.j, whilst level portion 24 of the same curve corresponds to the display state of this area. The rising and falling portions respectively 26, 28 of said curve correspond to the cholesteric-nematic phase change and the nematic-cholesteric change of the liquid crystal respectively and consequently to the passage from the undisplayed state to the displayed state and vice versa.
At present, several control processes for a liquid crystal matrix display are known, which make use of the transition effect of the cholesteric-nematic phase of said crystal.
In one of the known processes, the sensitization of area x.sub.i y.sub.j of the liquid crystal, i.e. the obtaining of one of the states, i.e. displayed or undisplayed, is brought about by the transmission on line x.sub.i for a time t.sub.1 equal to r.tau., in which r is an integer and .tau. is an elementary time interval useful for control purposes, an electric blanking signal having an amplitude well above the high threshold voltage V.sub.H of the liquid crystal followed by an electric addressing signal of said row, for a time t.sub.2 equal to .tau.. The integer r is dependent on the transition speed between the two phases of the liquid crystal used. Its value is a few units, generally 1, 2 or 3. Time .tau. corresponds to the minimum time necessary for the nematic-cholesteric phase change of the liquid crystal (passage from the nematic phase of the cholesteric phase). These electric signals are generally alternating signals with a mean zero value.
FIG. 3 shows as a function of time, a control signal of row x.sub.i, Va corresponding to the effective voltage of said signal. Part 29 of the signal corresponds to the blanking signal and part 31 thereof to the row addressing signal.
Moreover, to column y.sub.j is applied an electric addressing signal, particularly an alternating signal with a mean zero value having an effective value which is generally equal to that of the addressing signal of row x.sub.i, said signal being either in phase or in phase opposition with the addressing signal of row x.sub.i during the addressing time t.sub.2 thereof. FIGS. 3b and 3c show as a function of time, the addressing signal of column y.sub.j, respectively in phase and in phase opposition with the addressing signal of row x.sub.i, V.sub.B corresponding to the effective voltage of said signals.
When the signals applied to row x.sub.i (FIG. 3a) and column y.sub.j are in phase (FIG. 3b), these signals having equal amplitudes, the potential difference V.sub.c at the terminals of the liquid crystal in then zero, i.e. lower than the low threshold value V.sub.B of said crystal (V.sub.B &lt;0). In this case, the undisplayed state of area x.sub.i y.sub.j is obtained. In the same way, when the signals applied to row x.sub.i (FIG. 3a) and column y.sub.j (FIG. 3c) are in phase opposition, the voltage V.sub.c at the terminals of the liquid crystal is then equal to 2V.sub.O, if V.sub.O represents the effective value of said signals. Value V.sub.O is chosen in such a way that the voltage 2V.sub.O at the terminals of the liquid crystal exceeds the high threshold voltage V.sub.H of the crystal, which makes it possible to obtain the displayed state of area x.sub.i y.sub.j.
In accordance with the sequential control of a matrix display, the p rows are successively controlled and the q columns are simultaneously controlled in order to bring about the appearance on the display of an image, or an alphanumeric character, defined point by point.
In an article by KARL-HEINZ WALTER and MIROSLAV KARL TAUER, which appeared in the IEEE Journal of Solid-State Circuits, Vol. SC-13, No. 1, February 1978, entitled "Pulse-Length Modulation Achieves Two-Phase Writing in Matrix Addressed Liquid-Crystal Information Displays", a control process of this type was described.
FIG. 4 shows the response curves of area x.sub.i y.sub.j of the liquid crystal as a function of the preceding sensitizations. These curves give the light intensity (I) transmitted by the liquid crystal area as a function of time. The rising part 30 of the two curves O and P corresponds to the cholesteric-nematic phase change of the liquid crystal (passage from the cholesteric phase to the nematic phase), said phase change taking place during the blanking cycle t.sub.1. It should be noted that the time for obtaining this phase transition is relatively long, so that it must be carried out during the blanking cycle t.sub.l of row x.sub.i. The level portion 32 of curve O corresponds to the displayed state of area x.sub.i y.sub.j obtained when the signals applied to row x.sub.i and column y.sub.j are in phase opposition, whilst level portion 34 of curve P corresponds to the undisplayed state of area x.sub.i y.sub.j obtained when signals are applied in phase to row x.sub.i and column y.sub.j. The falling portion 34a of curve P corresponds to the nematic-cholesteric phase change of the liquid crystal.
In such a control process, during the blanking time t.sub.i, the q areas of row x.sub.i are in the displayed state, in view of the fact that the q columns of electrodes are simultaneously controlled. Thus, a white line appears over the entire length of the display. During the sequential addressing of all the rows, i.e. the addressing of the rows one after the other, a while line passes from top to bottom of the display. This white line, which appears whenever it is wished to modify the state of area x.sub.i y.sub.j is very unpleasant for the person looking at the display, particularly with respect to the areas thereof which it is wished to maintain in one of these states, namely displayed or undisplayed.